INOM EXAMENSARBETE MASKINTEKNIK,AVANCERAD NIVÅ, 30 HP

, STOCKHOLM SVERIGE 2016

Ultrasonic Testing of INCONEL Alloy 600

TORSTEN ENGSTRÖM

KTHSKOLAN FÖR TEKNIKVETENSKAP

Ultrasonic Testing of INCONEL Alloy 600

Torsten Engström

Report

Master thesis

Supervisor: Milan Poznik

Stockholm, 2016

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Abstract

In this report the attenuation of a core shroud mock-up is measured. The attenuation is measured for

two welds in different angles and for different frequencies. These measurements can then be recreated

in the simulation software CIVA for the 0º probes. The difference in the attenuation between the welds

is explained by transient heat flow simulations done in ANSYS. These show that due to geometry

differences the temperature gradient, which is one of the main factors of the growth directions for the

columnar grains, the heat distribution for the welds differs and thereby also the columnar grains and

the attenuation.

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Preface

This thesis, and the work it presents concludes my masters degree

at the Nuclear Engineering at the Swedish Royal Institute of Technology.

I would like to thank my supervisor Milan Poznic for his guidance and patience in spite of his

great workload. I am also grateful to all the other assistance I have gotten during the last

months, especially to Richard Samuelsson, Lars Skoglund and Johan Darth for their help with

measurements, simulation software and parts manufacturing.

I would also like to thank my wife for all the support she has given me throughout my

education.

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Table of contents

Abstract ...................................................................................................................................... 3

Preface ........................................................................................................................................ 4

1. Introduction ............................................................................................................................ 6

2. Problem Description ............................................................................................................... 8

3. Background ............................................................................................................................ 9

3.1 NDT .................................................................................................................................. 9

3.2 Welds .............................................................................................................................. 12

3.3 Wave propagation ........................................................................................................... 17

3.4 INCONEL 600 ................................................................................................................ 20

3.5 Attenuation ..................................................................................................................... 21

3.6 Anisotropic materials ...................................................................................................... 22

3.7 Piezoelectric transducers ................................................................................................ 24

3.8 The effect of defects ....................................................................................................... 25

4. Method ................................................................................................................................. 26

4.1 Simulations of anisotropic materials .............................................................................. 26

4.1.1 CIVA ........................................................................................................................... 27

4.2 ANSYS ........................................................................................................................... 29

4.3 Calibration ...................................................................................................................... 30

4.2 Experimental Setup ......................................................................................................... 34

4.2.2 ANSYS Setup .............................................................................................................. 38

5. Results and discussion .......................................................................................................... 39

5.1 Attenuation of the welds ................................................................................................. 39

5.2 CIVA Simulations .......................................................................................................... 45

5.3 Possible reasons for the deviation in attenuation ............................................................ 47

6. Conclusions .......................................................................................................................... 52

7. Future work .......................................................................................................................... 53

7.1 Distinguish the primary cause of the columnar grain growth direction ......................... 53

7.2 Enable angled simulations .............................................................................................. 53

References ................................................................................................................................ 54

Appendix A .............................................................................................................................. 56

Appendix B .............................................................................................................................. 61

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1. Introduction

Today’s nuclear industry is pushing its expected lifetime further and further away, due to

economic reasons and due to the need of a power source that can provide the grid with a

continuous base power. The extension of the operating time does however come with certain

costs, one of them is the increased strain and stress of the materials. To make sure that the

nuclear power production is safe the nuclear industry must make sure that the material that

might have been designed to last for a lifetime of 30 years can operate for approximately

another 30 years without endangering its structural integrity.

One of the materials used in Swedish power plants is INCONEL alloy 600. This material can

be used in boiling water reactors core shrouds or pressure water reactors steam generator

tubing. However recent discoveries have shown that this material shows unexpected

tendencies of stress corrosion cracks, fretting and denting. It is therefore very important to be

able to examine if the material has developed any operating induced cracks or defects. To

investigate this non-destructive testing is performed.

Non-destructive testing (NDT) is a way of testing the material without causing any damage to

it, this means that the parts to be tested can be tested in their operating place. Any time delay

can be costly for both the owners of the facility and the company performing the testing.

There are several ways of doing this each with its own benefits and disadvantages. This report

will focus on ultrasonic NDT.

Ultrasonic testing is a way of testing if the material or welds contain any cracks or defects by

transmitting ultrasonic waves into the material and then using a receiver to collect the

returning signal thereby making it possible to detect cracks and defects in the material.

A problem that arises with this method is that even though it works very well for

homogeneous materials it might not work as well for materials like INCONEL 600, which are

anisotropic and have columnar grains. The columnar grains cause the ultrasonic signal to

distort and diverge. It is therefore very difficult to test these types of materials and the result

of the tests will depend on many different variables such as angle, frequency and size of the

probe, but also on how the welds were performed and the geometry of the test object. This

makes it very time consuming to find a configuration of probe and frequency that works with

the material and chances are that the same configuration will not work for the same material if

the welding parameters differs.

Performing non-destructive testing of austenitic steels is problematic, the difficulty arises due

to that the weld material both has an unpredictable anisotropy and inhomogeneity. Thus

making it difficult to predict how the ultrasonic waves will behave when traveling through the

material. During the solidification of the material, the austenitic phase forms long columnar

grains that will grow in different directions depending on the temperature gradient and since

the temperature gradient will depend on heat input, geometry and pre-heating temperatures

these grains will change along the structure.

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The microstructure of the welds will effect the ultrasonic wave phase velocity, group velocity,

attenuation and grain back scattering. Due to many unknown material parameters, and a lack

of validated modelling tools for wave propagation in these materials, Ultrasound testing

technique development can be very time consuming. Typically, for every component that is to

be inspected in a nuclear power plant, test blocks with respective flaws needs to be fabricated.

For these test blocks both the welding procedure and the materials are known and match the

actual parts. But since the microstructure of the material is highly complicated, even qualified

guesses for what probe size, frequency and incidence angle to be used often fail.

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2. Problem Description

The aim of this report is to measure the attenuation of a INCONEL 600 test block, both in the

foundation material and in the welds. The measurements will be performed on a replica of the

core shroud of the Oskarshamn 3 reactor. This anisotropic steel is to be measured at 0, 25 and

60 degrees using longitudinal waves. The base material is then to be normalized with the

material in the welds, by doing so the additional attenuation that occur when sound is

traveling through the weld can be measured. If the attenuation varies over the test block the

aim is to see why those variations are occurring.

The welds will then be simulated with CIVA EXTNDE with the aim of recreating the

attenuation of the material within the program. If possible the simulation program can be used

to see what probes, frequencies and angles are able to penetrate the welds.

Instead of measuring the material constants and trying to preform accurate simulations that

are dependent on the varying and unpredictable material constants, the attenuation of the

material can be measured.

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3. Background

3.1 NDT

NDT is a way to evaluate the properties of a material without causing any damage to it, and is

often used for parts that are in operation.

There are several types of NDT methods, among the most commonly used ones are [1]:

- Magnetic Particle Testing (MP)

- Liquid penetration testing (PT)

- Radiographic Testing (RT)

- Electromagnetic Testing (ET)

- Visual Testing (VT)

- Ultrasonic Testing (UT)

MP is a method for detecting surface cracks and cracks directly beneath the surface in

ferromagnetic materials. A magnetic field is applied to the structure and if a crack is present

the magnetic field will change. If ferrous particles are applied to the material the particles will

be drawn to the area where the change in the magnetic field is.

PT is another method for detecting surface cracks that can be used on all non-porous

materials. The penetrant is applied to the material, the piece that is tested will then soak in the

penetrant for approximately 5 to 30 minutes. The excess penetrant is then removed. After

removing the excess penetrant, a developer is applied. The developer will draw the penetrant

out from cracks and thereby make them visible.

RT is a method that uses high energy photons or neutron to penetrate materials, radiation

sources can be 60 ,Co 192 Ir or an X-ray tube [2]. When radiating the material, the absorption of

the radiation in the material will vary depending on the structure and by using an X-ray film

on the opposite side of the radiation source the amount of radiation that has travelled through

the material can be visualised. The obtained images may however be blurry and cracks might

be difficult to detect.

ET can use either electrical currents, magnetic fields or both to observe the materials

response, the most commonly used method is to use eddy current. For eddy current a coil

carrying an AC current is placed near the material that is going to be tested. The alternating

current will generate a magnetic field and variation in the magnitude and phase can be

monitored. This method can be used on rough materials since there is no need for contact with

the material.

VT is where the test piece is examined with the eyes and the inspector try to find any visible

cracks. To make it somewhat easier to find cracks the lighting conditions need to be very

good.

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UT is when cracks are detected by transmitting ultrasonic waves through the material. This

report will focus on aspects of UT. Ultrasonic testing is a NDT method that is based on the

propagation of sound through materials, the frequency span that is above the human hearing

range (20 – 20 000 Hz) and ranges from 5 kHz up to 15 MHz [1].

Ultrasonic testing is a great method to use for testing material that can be both thin and thick.

Since the equipment is small the testing can be done in areas that are hard to reach due to a

lack of space. Testing can also be done with the equipment submerged in water which can be

applicable to the nuclear industry. Since performing the test themselves does not require

advanced movement testing can be done in radioactive environments by using robots to hold

and handle the probes during the scanning while the personnel is situated at a safe distance

away from the radiation. Ultrasonic testing is therefore an excellent method for testing nuclear

power plants.

The testing can be done by two different methods, transmission or impulse scanning [3]. In

transmission scanning a probe is placed on either side of the material, one transmitter and one

receiver, as shown in Figure 1. When measurement is done by transmission the transmitter

will emit the ultrasound, the sound then travels through the material and will be detected by

the receiver. If the sound encounters a crack the crack will reflect the sound and thereby

reducing the transmitted signal.

Figure 1: Inspection done with transmission. A crack between the transmitter and the receiver reflects the sound causing the

receivers signal to be weaker.

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When using the impulse-method the sound is reflected back to the probe from the backwall.

For impulse scanning only one probe is used, for both the transmission and the receiving of

the signal, as shown in Figure 2. Here the sound is emitted in impulses where the probe first

emits the signal for user defined amount of time, then switches to receiving the signal. If the

material contains a crack the sound will be reflected back to the probe. This signal arrives to

the probe before the signal that is not interrupted by a crack and therefore the crack can be

detected.

Figure 2: Inspection done with the impulse method. When the sound encounters a crack the signal is reflected back to the

probe. The time it takes for this signal to reach the probe is shorter than the reflection from the backwall.

To be able to emit sound the probe contains a piezo-electric element, which will be further

discussed in section 3.7 piezoelectric transducers.

To transfer the sound between the probe and the material to be tested a couplant is used.

Without a couplant the acoustic impedance mismatch between the solid and air becomes

large, this causes nearly all the energy to be reflected and the transferred signal into the

material becomes very small [4]. The couplant to be used for the measurements depends on

the wave type. In this report water will be used since it is easy to apply and is able to transfer

longitudinal waves.

To make result evaluation easier results can be visualized in a couple of different ways. The

A-scan shows the pulse that is generated into the object and displays the echo as a function of

time/depth. Where the amplitude is plotted on the y-axis and time/distance on the x-axis. To

assess the size of the discontinuities with an A-scan the signal from an unknown reflector

needs to be compared with the signal from a known reflector. The B-scan is combining the A-

scan with the movement of the probe along the test block. It shows the travel time of the

ultrasonic signal is represented as a displacement on one axis and the movement of the

transducer is represented on the other axis. This gives an image of the material where holes

and defects becomes more visible due to the clear reflection. A B-scan can be viewed in

Figure 17. Another way to view the results is the C-Scan, a two dimensional data presentation

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where the data is view as a top or planar view of the test block. This can be resembled with an

X-ray and the colours displayed represent the depth at each point or the signals amplitude.

The echodynamic view is a mixture of the sound path, probe position and the amplitude.

3.2 Welds

Welding is a very common method during construction. When two materials are forged

together with a weld, the heat source will interact differently with different parts of the

material. This will create three different regions, these are the fusion zone, the heat affected

zone (HAZ) and the unaffected base material. [2]

In the fusion zone the temperature exceeds the liquidus temperature so the material

experiences both melting and solidification during the welding. The microstructure of the

fusion zone will depend on many parameters, for instance on how the heat source interacts

with the material, mass flow of shielding gases, preheating temperature and dissolution of

gases among other things.

In the HAZ the temperature is not high enough to melt the material but the structure of the

material will change. The HAZ limit towards the unaffected base material is unclear, this is

because the temperature gradient decreases with increasing distance from the weld, and the

limit will depend on what type of material that is welded. However, for general construction

steels the limit is where the maximal temperature reached is 600 ºC [2]. Within the HAZ the

microstructure will change with the distance from the centre of the fusion zone and the heat

affected zone is therefore divided into four different parts, where the part closest to the fusion

zone will undergo a complete change in the microstructure and the part that is the furthest

away will experience insignificant changes to the microstructure. This means that the grain

size will also vary along the HAZ [7].

For austenitic steels the solidification process forms long columnar grains, and the direction

of the grains will depend on the temperature gradient. An example of this is shown in Figure

3. The diameter of the grains will not be constant, they can vary in range from 20 µm to 3

mm. [5,6] The large variation of grain size is due to the difference in the welding methods,

welding techniques and the solidification process.

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Figure 3: The columnar grains of an austenitic steel weld. [8]

The most commonly used methods for welding austenitic steel is [9]:

- Manual metal arc (MMA)

- Metal inert gas (MIG)

- Submerged arc (SAW)

- Tungsten inert gas (TIG)

are many factors that will influence the choice of welding method, for instance, number of

welds to perform, which standard that needs to be met, thickness of the material that is to be

welded, solid mechanical demands and cost. By using different welding methods, the heat

input into the weld will change which will affect the microstructure, this is especially true if

the weld is done by using multiple strings. If multiple strings are used, then the direction of

the temperature gradient will affect whether the dendrites grow on the primary dendrites or if

they will form in a new direction [5].

Difference in the solidification process can be due to different degrees of preheating.

Preheating the structure is done if there is a risk of hydrogen cracking, cracks in the weld

which appear below 200 ºC within minutes up too hours from when the weld was made.

Hydrogen cracks are mainly caused by three factors. The steels curability, hydrogen level and

internal tension [10]. The risk for hydrogen cracks in austenitic steels is lower than for ferritic

steels due to a higher solubility of hydrogen in the austenitic steels.

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Different geometries and pre-heating temperature will alter the cooling time. The cooling time

for a weld is usually measured in 8/5t which is the time it takes for the material to cool from

800 ºC to 500 ºC.

The possibility to detect defects within the weld and the areas adjacent to the weld are

dependent on the welds root shape, root sagging and the shape of the weld [11].

The welds tested in this report are performed using MMA. Each of the weld is done with

several strings, since the two pieces that are forged together are too large for a single weld to

be able to forge them together. In fact, the welds to be tested are so large that weld 68 is a

composition of 76 strings and weld 69 is composed by 47 strings, see Figure 19.

The welding positions are different for the two welds. For weld 68 the welding position is PA

and weld 69 is done with position PC, see Figure 4.

Figure 4: Welding positions according to EN ISO 6947 [12]

The welds are performed according to Table 1 and Table 2.

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Table 1: Welding parameters for weld 68.

Weld 68

String Nr Preheating temperature Amplitude (A) Voltage (V) Polarity

1-76 RT (Room temperature) 90 28 +

Table 2: Welding parameters of weld 69

Weld 69

String Nr Preheating temperature Amplitude (A) Voltage (V) Polarity

1-8 RT 90 24.3 Unknown

9-11 RT 119 25.6 Unknown

12- RT 123 25.9 Unknown

-23 RT 117 24.8 Unknown

24-34 RT 117 24.3 Unknown

35-47 RT 156 27.4 Unknown

The advancing speed for both the welds is unknown.

The polarity, amplitude and voltage are parameters that affect the weld penetration and width,

so by altering these parameters the welds appearance and thereby solid mechanics can be

altered.

Figure 5: The effect of different polarities during welding [2].

Figure 6: The effects of changing the voltage during welding [2].

Figure 7: The effects on a weld by changing the current [2].

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The effects of changing the parameters of the equipment of a weld are shown in Figure 5,

Figure 6 and Figure 7. For weld 68 the same parameters are used for every string, but for weld

69 both the current and the voltage are altered throughout the weld. The main difference is

that the amplitude is increased for the later strings. The post processing of the welds consists

of grinding of the weld to flatten the surface.

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3.3 Wave propagation

Waves will travel through different materials in different ways. The description of how a

wave propagates through a solid start with a simple harmonic wave, which can be expressed

as:

( , ) cosx

u x t A tc

(1)

where u is a particles displacement, A is the amplitude, is the phase angle, c is the velocity

and is the angular frequency defined as

2 f (2)

where f is the frequency.

Depending on whether sound is traveling through a solid or a fluid the ultrasonic waves will

behave differently. For a fluid like air sound will travel by longitudinal waves as shown in

Figure 8. For a longitudinal wave the propagation direction is the same as the direction of the

particles motion. This is the same type of wave as in the sound we hear. This wave type can

exist in solids, liquids and gases.

Figure 8: Shows a longitudinal wave for which the propagation direction and particle movement direction are the same. [14]

Another type of wave is the shear wave, which can only exist in a solid. This is due to that the

bonds between molecules in solids are much stronger and also because the molecules are

packed tighter [13].

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For a shear wave the particle motion is perpendicular to the waves direction of propagation as

shown in Figure 9.

Figure 9: The Propagation and particle direction of a shear wave [14]

One factor that makes ultrasonic testing quite complicated is the process when a wave strikes

a surface or a boundary. If the wave strikes at the boundary at a right angle towards the

surface some of the wave will be reflected and some of it will be transmitted as shown in

Figure 10.

Figure 10: Part of the wave is transmitted through the boundary and the other part of the wave is reflected, the amount of

transmitted and reflected wave will depend on the material. [14].

However, if the wave strikes the surface or boundary at an angle both a longitudinal and a

shear wave might occur as shown in Figure 11.

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Figure 11: A incoming longitudinal wave reflected and divided into a shear and longitudinal wave. [14]

This phenomenon is called mode conversion and occurs if an incoming longitudinal wave

encounters a boundary at an angle, then some of the particles in the material might start to

move in the transverse direction, and this starts a shear wave.

This becomes a major problem with ultrasound, since this can occur every time a wave

encounters a material with a different impedance and the wave strikes the boundary with an

angle. This will lead to the wave being reflected many times, weakening the signal and

causing poor signal to noise ratio (S/N ratio) and can also cause false positives i.e. the signal

appears as if there is a crack in a material due to the reflections even though there is no defect.

The acoustic impedance is the relationship between the density and velocity of a material and

is for a longitudinal wave defined as

L LZ c (3)

where Lc is the longitudinal velocity and is the density. This will affect the reflection and

transmission of the waves when they encounter different materials. A high impedance

difference will lead that the majority of sound is reflected. A smaller impedance difference

will cause the sound to be transmitted.

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3.4 INCONEL 600

The ultrasonic waves will behave different depending on what material they travel through. In

this report the base material is INCONEL 600 and the added material to the weld will be

INCONEL 182.

INCONEL 600 is commonly used in nuclear power plants and can be used for example in

core shroud s or for the steam generator tubes. The reasons for INCONEL 600 being suitable

for the purpose is that INCONEL is a Cr, Ni and Fe alloy with good corrosion and heat

resistance. The material is also able to withstand high levels of radiation and does not show

signs of intergranular stress corrosion cracking for neutron fluences below 24 25 10 /n m [15].

The limiting chemical composition of INCONEL 600 is shown in Table 3.

Table 3: Limiting chemical composition [17]

Material %

Nickel (plus Cobalt) 72.0 min

Chromium 14-17

Iron 6.00-10.00

Carbon 0.15 max

Manganese 1.00 max

Sulfur 0.015 max

Silicon 0.50 max

Copper 0.50 max

One of the problems with ultrasonic testing of INCONEL 600 is that it has columnar grains,

that are long, thin and coarse. They primarily grow in one direction which is dependent on the

temperature gradient, meaning that if the temperature gradient differs along the structure the

grain growth direction may differ. Due to the local thermal gradients the columnar grains can

be tilted in both the welding direction and in the plane perpendicular to it [16]. Two

neighbouring columnar grains will create a boundary between them that can scatter the

ultrasonic signal and make detection of any defects very difficult. This boundary can also

cause the signal to go through a mode conversion.

Another problem with INCONEL 600 that holds true for all austenitic steels is that it is

anisotropic. This means that due to its microstructure, the properties of the material will vary

with direction. A result of this is that the sound velocity varies depending on direction.

The speed of sound in a material is defined as

M

v

(4)

Where M is the appropriate modulus (depending on wave type) and is the density [18].

Another effect of the columnar grains is that the attenuation of sound will increase.

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The production procedure has a known effect on the microstructure of the material, however

in this case the specific production method for the test block is unknown.

3.5 Attenuation

One factor limiting the possibilities of ultrasonic testing is high levels of structural noise and

attenuation within austenitic steels [21]. Attenuation is the reduced intensity of sound with

distance travelled, and will depend on spreading of the wave, scattering and absorption.

Attenuation is defined as the ratio between two amplitudes

1

2

20logA

LA

(5)

Where 2A is the amplitude with attenuation, 1A is without and L is the attenuation in dB.

For ordinary steels with a low amount of alloys the attenuation is low. However, for high

alloy materials or materials that have been fabricated through casting or rolling the attenuation

can become higher.

Since water is used as a couplant this water will also contribute to the attenuation of the

signal. Even though there are two layers of water (one on each side) the water layers can be

considered to be thin. The attenuation due to water is around 0.2 dB/mm [14] for frequencies

around 1 MHz and this signal loss can therefore be considered to be so low that the effect of it

can be disregarded.

One of the most important mechanisms of attenuation when using ultrasound in

polycrystalline materials is the scattering of sound due to grains. This scattering will depend

on the frequency used during the testing but also the grain size of the material [19].

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3.6 Anisotropic materials

To understand the difference between wave propagation in isotropic and anisotropic material

it is necessary to define the stress in an anisotropic material

ij ijkl klC (6)

where ij is the stress on the i:th face in the j:th direction, ijklC is the fourth rank elastic

stiffness tensor and kl is the elastic strain tensor defined as

, ,

2

k l l k

kl

u u

(7)

Placing equation 6 into Newtons law

2

2

ij i

j

u

x t

(8)

yields

2

2

kl iijkl

j

uC

x t

(9)

where iu is the displacement of a particle within the body in the i:th direction and is the

density. Now placing equation 7 in equation 9 and reducing the equation due to symmetry

with respect to k and l, the equation becomes

2 2

2

i lijkl

j k

u uC

t x x

(10)

finally considering a plane harmonic wave of the form

( ), i kx tu x t Ae (11)

where u is the displacement, A is the amplitude, is the angular frequency and k is the wave

number defined as

kc

(12)

Equation 11 is the complex notation of equation 1. Differentiating equation 11 twice and

using equation 10 and 11 the final equation can be written as

2[ ][ ] 0ik ijkl j l kc C n n u (13)

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Where ik is the Kronecker delta, jn and ln represent the wave fronts propagation direction

and ku is the displacement in the k:th direction. This equation is called the Christoffel

equation. This equation can be viewed as an eigenvalue equation and solving this gives three

eigenvalues that relates to three different velocities of propagation [20].

A consequence of this is that for an anisotropic material there are three different wave types.

Generally, these waves will not be purely longitudinal or purely shear waves, with exception

of certain directions, but they will be quasi-longitudinal and quasi-shear waves. A difference

between these waves and waves in isotropic material is that the phase and group velocity do

not coincide.

When the wave encounters a boundary caused by a columnar grained material, the wave is

transmitted into three different waves, causing more dispersion of sound. The signal will be

even lower when the sound reaches the receiver. Previous attempts to address the problems

associated with columnar grains include measuring the sound velocities in different direction

for the material and through that determining the constants in the elastic stiffness tensor.

However, though this works fairly well for the exact position where the measurements have

been done, it does not work in practice since the material properties will change when the

probe changes its position and therefore also the elastic stiffness tensor will change.

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3.7 Piezoelectric transducers

Piezoelectric transducers are used to convert the electric energy into acoustic energy. The

transducers are able to convert an electric pulse into a mechanical vibration, but also able

convert mechanical vibrations back into electrical pulses. If two electrodes are connected to

each side of a thin piezoelectric material and an electrical field is put between the electrodes

the molecules in the material will align themselves with the electric field as shown in Figure

12. When the molecules align themselves with the electrical field the structure will change

shape, and the thickness of the thin layer can be varied according to the variation of the

electrical field and therefore waves can be created [22]. It is also important to note that this

effect can be reversed.

Figure 12: Applying a charge to the piezoelectric material.[23]

The wave generated from the piezoelectric material will not originate from a single point but

from several points along the surface, causing the different waves generated to cause both

constructive and destructive interference. The piezoelectric material is fragile and therefore

probes need to be handled with great care. Dropping a probe could cause it to break.

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3.8 The effect of defects

Discontinuities and defects are obstacles in the ultrasound’s directions of propagation through

the material. Depending on the size of the wavelength and the size of the defect or

discontinuity there are three main cases [24]:

- The size of the defect is much larger than the wavelength. This causes the signal to

reflect clearly in a well-defined beam.

- The size of the defect is of the same size as the wavelength. When the wave is

reflected the beam becomes divergent in a cone-shape.

- The size of the defect is much smaller than the wavelength. This causes the beam to be

reflected in all directions.

So to be able to locate a defect the signal needs to have a wavelength that is much smaller

than the defect.

When measuring through transmission there are a few possible scenarios that can affect how

the sound travels through the material as is shown in Figure 13. If the material does not

contain any defects and the material is homogenous the sound will simply travel through the

material. However, if there is a lack of homogeneity the signal will be scattered the distance

the sound travels through the material will increase, as will the time it takes for the material to

reach the other side.

If the sound encounters an air filled crack the signal will not travel through the crack but can

be reflected at the edge, resulting in a greater travel path for the signal.

The defects are placed in the test block in order for a company to see which frequencies and

angles are the best a locating the defects for this specific type of material.

Figure 13: Possible sound paths when measuring through transmission [24].

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4. Method

4.1 Simulations of anisotropic materials

Simulations of the propagation of sound can help a lot in understanding how the sound travels

through the material and the weld. The use of simulation allows for properties to be altered to

match real life circumstances.

Simulations of elastic wave propagation in anisotropic material use one of the three following

methods [5]:

- Numerical

- Analytical

- Approximate

The numerical approach uses the finite element method to describe the propagation of sound.

A problem with this method is that in order to describe the propagation of sound the element

size would have to be very small. This is due to that fact that since ultrasonics uses

frequencies at several MHz, the wavelength can be calculated by

v

f (14)

and the speed of sound for longitudinal waves in austenitic steel can be around 5800 m/s.

Finitie Element Programs (FEM) program developers like COMSOL Multiphysics

recommend that simulations of ultrasonics need an element size of 5-6 elements per

wavelength. This would require a tremendous computer power and would take a very long

time to solve even simple problems. Therefore, the FEM approach is not suitable for

ultrasound simulations. The analytical model uses advanced mathematical approaches where

all the characteristics of the material need to be known to complete the computations. The

approximation approach uses partly analytical methods and partly numerical methods, making

it much faster than the numerical method. This is the basis for the program used in this report.

27

4.1.1 CIVA

CIVA is a program that is designed to simulate ultrasound, eddy current and radiography. The

French Atomic Commission has been involved in the evolution of CIVA. Within the program

simulations of different frequencies, incident angles, probes and materials can be made. It is

also possible to insert defects into welds and study the reflection of the sound. CIVA’s wave

propagation modelling is based in an integral formulation of the radiated field and applies the

so-called pencil method. [25]

In the pencil method a point source radiates a spherical wave. This wave will be considered as

a plane wave when the distance r between the source and the observational point is large.

The plane wave will be characterized by a vector that is defined by the point source and the

observational point. The plane wave’s intensity will decrease with distance as a function of .r

A pencil is then a number of rays that is sent out from the point source, with the rays slightly

diverging with distance, as shown in Figure 14.

Figure 14: Diverging waves that sums in the target and S in the cross-section of the target. [26]

The divergence factor µ can be calculated by using the cross section dS and the initial solid

angle .d The divergence can be calculated by

dS

µd

(15)

To be able to describe the propagation of the cross-section, the vector ( , , , )x ydx dy ds ds is

defined. Where dx and dy represent the intersection of the propagating ray with the plane

tangent to the wave front, xds and yds is the projection of the slowness vector of the paraxial

ray on the plane as shown in Figure 15.

28

Figure 15: The mathematical description of the propagating pencil. [26]

The slowness vector is the vector that is perpendicular to the wave front and has a magnitude

that is inverse to the velocity.

The pencil vector of the source and the pencil vector of the propagating rays are linked by

'

A B

C D (16)

Where ' is the pencil vector at the source and A, B, C and D are 2 by 2 matrices.

By using these equations one can find the configurations necessary to describe propagation in

isotropic or anisotropic media as well as the refraction and reflection at interfaces.

This makes the method very suitable for ultrasonic simulations.

In order to make sure that the simulation and the measured values do not differ from one

another, verifying measurements should be made. This will be further discussed in section 4.3

Calibration.

If the anisotropy of the material is known the weld can be divided into several sub-categories

where the anisotropy is set for each individual part of the weld [27]. This however requires

extensive knowledge of the microstructure.

29

4.2 ANSYS

ANSYS is a finite element program. Even though this program is not suitable to perform the

simulations of the propagation of the ultrasonic waves it is a good program to perform

transient heat simulations. By knowing the parameters for the welding and the weld method,

the heat input Q [kJ/mm] can be calculated.

60

1000

k U IQ

(17)

Where k is the thermal efficiency, which is dependent on the welding method, U is the

voltage and I is the current. With this simulation the heat distribution that is caused by the

weld can be visualized.

For an ANSYS thermal transient simulation a certain uniform heat input is imposed on a

surface. Prior to the heat input the surface is of uniform temperature. A thermal analysis in

ANSYS basically follows the same procedure as the steady state thermal analysis [28], the

main difference being are that the transient analysis are functions of time.

The process solves a heat transfer equation

q k T (18)

where q is the local heat flux density, k is the conductivity and T is the temperature

gradient.

One of the drawbacks with using ANSYS is that for the ANSYS academic license the number

of nodes available for the thermal transient calculations is limited to 32 000. So using large

models will cause the use of large elements. Large elements will in turn lead to a reduced

accuracy of the model, but since the ANSYS analysis only will be used as an illustration of

the heat propagation and no precise values for specific places are required, the transient

analysis will be sufficient even with the element limitations.

30

4.3 Calibration

Before performing any of the measurements the equipment needs to be calibrated.

The calibration is usually done on a calibration block, but can also be done on the test block.

It is important to know material parameters like thickness and sound velocity when

calibrating.

The calibration block is a block with known material properties, dimensions, longitudinal and

transversal sound velocities. The block has 8 side-drilled holes as shown in Figure 16. The

holes are there because they will reflect the sound. Some defects will reflect more sound than

others and in an optimal setup the defect in the calibration block should look the same as the

defects that are being searched for. Creating the same type of defects that is being searched

for is however difficult, and due to this fact it is common to use a block where holes are

drilled. The drilled holes are often better as a reflector of sound due to that the surface

becomes smoother and flatter. Another reason to use drilled holes is that it would be much

more expensive to produce realistic defects.

The eight holes are located at different depths from the surface, allowing measurement of

ultrasonic signal reflection at different depths. The measured signal response from the holes

can then be compared with the simulated response. If the signal response is not the same, then

the simulation settings needs to be adjusted.

Figure 16: The CIVA simulated calibration block with eight side-drilled holes in red, and the probe in yellow. The green line

is the wave direction for the probe and the purple line is the probes path.

31

The validation measurements will be done with four different probes, of which two are

circular longitudinal probes with a diameter of 12.7 mm and 19.05 mm. The 12.7 mm probe is

called 30428 and the 19.05 mm probe is called 28490. The transmission angle for both of

these probes is 0º. To calibrate the signal strength for these probes the probe is placed on the

calibration block so that the signal reflects from the back wall. The gain is then adjusted until

the input signal is 80% of the maximum input strength.

Two rectangular probes will also be used. One probe is transmitting shear waves, has an area

of 640 mm2 and an transmission angle of 55°. This probe is called 15DV-01. The other is

transmitting longitudinal waves and has an area of 420 mm2. The transmission angle for this

probe is 60º and is called 15DX-01. To calibrate these probes, the back echo cannot be used

since the signal is angled. The probe is therefore calibrated towards a rounded corner in the

calibration block. The radius is set for the rounded corner and the gain of the signal is set

towards the response from the corner.

The B-scan for probe 28490 is presented in Figure 17.

Figure 17: B-Scan of the calibration block done with the probe 28490. The depth of the calibration block in mm on the Y-

axis, and the length distance in mm on the X-axis. The eight holes have clear reflection at their respective depth, and at a 100

mm on the Y-axis the reflection from the backwall is visible.

When comparing the measured values with the simulated values the amplitude drop between

each hole is compared, the first hole becomes the reference with an amplitude drop of 0 dB.

This is because of CIVA setting the first hole is as a reference with response amplitude 0, so

in order to compare the signals the same must be done for the measurements.

The amplitude drop of the remaining seven holes is in reference to the first hole.

32

Table 4:The measured amplitude drops, compared with the simulated amplitude drops. The first hole becomes the reference

hole and all the amplitude drops are in reference to that hole. The amplitude drop is measured in dB.

Hole

Nr:

Measured

values

probe

28490

CIVA

probe

28490

Measured

values

probe

30428

CIVA

probe

30428

Measured

values

15DX-01

CIVA

probe

15DX-01

Measured

values

15DV-01

CIVA

probe

15DV-01

1 0 0 0 0 0 0 0 0

2 -1.7 -1.7 -1.7 -4.2 -1.2 -2.1 -0.8 -0.7

3 -4.0 -4.3 -5.0 -7.3 -2.0 -2.8 -0.1 -1.7

4 -6.3 -6.4 -8.2 -10.2 -3.5 -4.2 -1.2 -2.9

5 -8.4 -8.6 -11.2 -12.2 -5.2 -5.9 -3.1 -5

6 -10.3 -10.2 -13.6 -14.1 -6.7 7.1 -4.5 -6.2

7 -11.9 -12.0 -16.1 -15.8 -7.8 -8.8 -6.6 -7.5

8 -13.5 -13.4 .22.8 -17.4 -8.5 -9.6 -6.8 -8.9

The 0º degree probe, 28490, is the probe that has the least difference between the simulated

and measured values. The second 0º degree probe, 30428, has a much greater difference, this

could possibly be because of this probe having a smaller diameter and therefore not being able

to penetrate the same depth in the calibration block. This is clearly visible when comparing

Figure 17 with Figure 18.

However, since the measurements are going to be done in transmission one probe needs to be

transmitting the signal and another probe needs to be used as a receiver. Since the 28490

probe works very well with transmitting the sound through the material this probe can be used

as the transmitter and the 30428 probe can be used as the receiver.

33

Figure 18: B-Scan of the calibration block done with the probe 30428. The depth of the calibration block in mm on the Y-

axis, and the length distance in mm on the X-axis. When comparing this B-scan with Figure 17 it is clearly visible that the

signal response decreases as the depth increases. Also the reflection from the backwall is lower than for probe 28490.

For both the longitudinal probe 15DX-01 and the shear wave probe 15DV-01 the measured

values are close to the simulated values.

34

4.2 Experimental Setup

The measurements will be performed on the test block shown in Figure 19. Two welds will be

examined, weld 68 and weld 69. These welds will be measured in transmission with

longitudinal waves and a frequency of 1, 2 and 2.25 MHz. The angles to be scanned are 0º,

25º and 60º, from the normal of the surface. The measurements are done in transmission due

to this being the most efficient way to measure the attenuation. If the attenuation is measured

using the same probe for transmission and reception the sound will reflect from the backwall

and thereby causing additional dispersion and mode conversion.

The welds will be scanned along the Y-axis as shown in Figure 19. The software used to

collect the data is UltraVision from ZETEC. Since measurements will be done with

longitudinal waves the coupling fluid on both the top and the bottom will be water.

Figure 19: A CAD model of the test block on which the measurements will be performed. The test block is a replica of the

core shroud of the Oskarshamn 3 reactor. The welds to be tested are weld 69 and weld 68. Weld 68 is divided into 3 parts as

shown in figure.

The test block is designed with the same material and welding method as the actual piece. The

block is produced with the purpose of developing a method for finding cracks in this specific

material. To help with this a number of defects are introduced along the welds within the

block. There are 12 induced defects in weld 68 and nine defects in weld 69. The defects are

spread out along the entire welds and their exact positions are shown in Appendix A.

35

To hold the probes in place during the scanning a special rig has been built, as shown in

Figure 20.

Figure 20: The rig holding the probes in their place during the scanning of the testblock.

A machine that is able to move the probe holder in any given direction controls the movement

of the probes. The machine is shown in Figure 21. The speed of the probes during the

scanning is set to 75 mm/s.

36

Figure 21: Machine controlling the movement of the probes. Here the scanning equipment is placed over leg 1.

Weld 68, legs 1-3 are manufactured in the same way, the only difference between the legs is

that leg 1 is smaller than leg 2 and 3, also the defects induced in the leg differs.

All of the probes are using longitudinal waves however the angled probes used are bigger and

are therefore less sensitive to any surface deviations.

Figure 22: Two of the angled probes used. These probes are 60º and 2 MHz. The dimension of the probe is 15 cm tall and 28

cm wide.

37

Figure 23: Two 0º probes. The smaller probe is 1.27 cm and the larger is 1.9cm. Both are transmitting a frequency of 1 MHz.

To measure the attenuation equation 5 is used. Here 1,A the unattenuated signal, will be the

signal that is measured in the base material. 2 ,A the attenuated signal, will be the signal that is

measured in the weld.

38

4.2.2 ANSYS Setup

Since the advancing speed is unknown for the welds the actual heat input for the weld is also

unknown. The deviation of welding parameters also creates a problem since the change in for

instance amplitude varies a lot with the different strings. Since it is known that the weld is

performed with MMA, a temperature of 1600° C can be applied to the surfaces on which the

weld is performed, to circumvent the problem. This temperature is lower than the maximum

temperature and does not represent the actual temperature that occurs during the welding

process. But since the purpose of the simulations is not to make a precise model of the

welding process but to simulate the heat distribution of the welding this approach serves its

purpose.

The heat input from the weld is considered as a single load where all of the heat is applied at

the same time thereby disregarding the fact that the weld is performed using a number of

strings.

The structure is not preheated which means that the welding occurs at room temperature.

Room temperature is assumed to be 22º C which is set as the ambient temperature.

The full simulation setup is shown in Appendix B.

39

5. Results and discussion

5.1 Attenuation of the welds

Since the probes are small and the surface roughness of the test block is fairly high, due to

surface grinding to flatten the welds, the contact between the probe and the test block is not

always perfect. The signal strength of the base material can have big variations as shown in

Figure 24.

Figure 24: The C-scan of weld 69. The scan is done with a 0º probe with a 1 MHz frequency.

In Figure 24 the weld is situated with the centre of the weld at 0 mm on the x-axis. This

means that the material to the right of 40-60 mm should be roughly the same.

But since springs are pushing the probe down towards the surface to ensure a good

connection, a dip in the surface can cause the probe to be angled in the wrong direction. This

will cause the signal to drop. This problem is more significant for the 0º probe since these are

smaller and thereby more sensitive to surface irregularities. The surface irregularities are most

likely there because of the post processing of the weld. By placing a ruler on the surface it

becomes obvious that the grinding is irregular and some places has lost more material than

others. This is however normal and is something that should be expected for welds.

To circumvent this problem, the attenuation needs to be measured where the contact of the

probe is good.

40

For the weld a contact problem becomes obvious since the signal drop is already so high that

when a contact problem arises the signal drops to almost 0. For the base material a part where

the contact is good needs to be selected in order to compare the weld with the base material. It

is also important to make sure that there are no induced cracks or defects in the selected area

since this would cause wrongful data.

Figure 25: The B-scan of weld 69 done with a 1 MHz 0º probe showing an area with good contact between probe and

material.

Comparing Figure 25 with Figure 26, both B-scans of different areas of the test block, it

becomes clear that the signal response is much better for Figure 25.

41

Figure 26: A B-scan of weld 69 done with a 1 MHz 0º probe showing an area with poor contact between the probe and the

testblock.

This is important since even if the contact is good there still deviations in the material. These

deviations can among other things be caused by the anisotropy of the material. Therefore,

when calculating the attenuation, the average of the base material will be used.

In order to get the average signal drop Ultravision calculates the average signal drop in a

selected area.

For the angled probes the surface area is much greater as shown in Figure 22 and Figure 23.

So for them the small surface differences do not cause as big of a problem as shown in Figure

27 where the base material’s signal response does not alternate in the same way.

42

Figure 27: The C-scan of weld 68-2 done with 1 MHz 25º probe. The weld centre is situated at 0 on the X-axis.

Since the contact is consistently good the base selection of which base material chosen is not

of great importance. The important part then is to make sure that there are no defects in the

selected part.

43

Table 5: Measured signal loss and attenuation for respective weld.

Weld 68-1

Frequency Angle Signal loss in base material

[dB]

Signal loss in the weld [dB]

Attenuation [dB]

1 0º -1.8 ±0.5 -34.4 ±0.5 25.626 ±0.009

1 25º -2.8 ±0.5 -17.5 ±0.5 15.918 ±0.014

2 60º -2.0 ±0.5 -13.7 ±0.5 16.714 ±0.013

2.25 0º -1.8 ±0.5 -16.7 ±0.5 19.349 ±0.011

Weld 68-2

Frequency Angle Signal loss in base material

[dB]

Signal loss in the weld [dB]

Attenuation [dB]

1 0º -1.7 ±0.5 -23.9 ±0.5 22.959 ±0.010

1 25º -2.3 ±0.5 -18.0 ±0.5 17.871 ±0.012

2 60º -2.8 ±0.5 -17.1 ±0.5 15.717 ±0.014

2.25 0º -1.3 ±0.5 -18.0 ±0.5 22.827 ±0.010

Weld 68-3

Frequency Angle Signal loss in base material

[dB]

Signal loss in the weld [dB]

Attenuation [dB]

1 0º -1.7 ±0.5 -29.4 ±0.5 24.758 ±0.009

1 25º -1.2 ±0.5 -19.4 ±0.5 24.172 ±0.009

2 60º -1.6 ±0.5 -17.3 ±0.5 20.679 ±0.011

2.25 0º -1.5 ±0.5 -24.0 ±0.5 24.082 ±0.009

Weld 69

Frequency Angle Signal loss in base material

[dB]

Signal loss in the weld [dB]

Attenuation [dB]

1 0º -1.7 ±0.5 -28.2 ±0.5 24.396 ±0.009

1 25º -1.0 ±0.5 -14.8 ±0.5 23.405 ±0.009

2 60º -1.5 ±0.5 -17.8 ±0.5 21.487 ±0.010

2.25 0º -1.8 ±0.5 -17.2 ±0.5 19.605 ±0.011

The signal response is lower for all measurements done with the 1 MHz 0º probes. This signal

drop is probably much higher than for the rest of the probes due to that these probes had the

biggest problem with contact. The high attenuation received for these probes can therefore be

a bit misleading.

The distance travelled is different for each of the probes since the incidence angle for the

probes differs. This means in order to compare the results the attenuation needs to be divided

with distance travelled.

44

Table 6: Attenuation with distance travelled accounted for.

Weld 68-1

Frequency Angle dB/mm

1 0 0.512 ±0.002

1 25 0.288 ±0.003

2 60 0.167 ±0.003

2.25 0 0.388 ±0.002

Weld 68-2

Frequency Angle dB/mm

1 0 0.448 ±0.002

1 25 0.325 ±0.002

2 60 0.157 ±0.003

2.25 0 0.456 ±0.002

Weld 68-3

Frequency Angle dB/mm

1 0 0.496 ±0.002

1 25 0.439 ±0.002

2 60 0.207 ±0.002

2.25 0 0.482 ±0.002

Weld 69

Frequency Angle dB/mm

1 0 0.458 ±0.002

1 25 0.424 ±0.002

2 60 0.215 ±0.002

2.25 0 0.392 ±0.002

It is clear that the attenuation differs with change in both frequency and angle. When

comparing the legs of weld 68, it becomes clear that there are similarities between the

changes in angle and frequency. The changes are not the same in value but they are following

the same pattern. Dropping or increasing with respective changes angle and frequency.

Since weld 68-1,2,3 is performed with the same methods and parameters the attenuation

should not differ much. The difference between 68-1 and 68-2 for the 1 MHz 0º probe is

0.064 dB/mm. One difference between leg one and two is that leg one is smaller which could

have affected the solidification process of the weld and thereby the microstructure of the

weld.

Leg two and three are however of the same size and the welds are performed with the same

parameters. This should mean that the attenuation should be roughly the same. However, the

difference between the two varies for the different angles and frequencies between 0.026

dB/mm to 0.114 dB/mm. Even though this does not sound like a lot it should be remembered

that the decibel scale is not linear. A drop of – 6 dB is half the sound pressure so even this

small difference in attenuation can have a big significance when scanning a large structure.

It should also be noted that the difference between the attenuation is higher for all angles and

frequencies for weld 68-3.

45

This difference is likely due to difference in the columnar grain direction and a difference in

the grainsize. Since it is the same method and welding parameters it can be assumed that the

temperature gradient, which is the cause of grain growth direction, should be about the same

as well. However, due to the design of the test block this might not be the case as further

discussed in section 5.3 Possible reasons for the deviation in attenuation.

5.2 CIVA Simulations

The CIVA transmission simulations are only possible to perform for the 0º probes as shown in

Figure 28. For the angled probes CIVA is not able to perfectly align the probes and is

therefore not able to perform the simulations.

Figure 28: CIVA setup of measurements done in transmission the setup shows the simulations performed on weld 69.

46

CIVA also sets the reference for any simulation to 0 as shown in Figure 29. This reference is

also set according to the weld. The simulations made in CIVA are beam computations, these

simulations calculate the amplitude and the beam path as shown in Figure 31. Since the

attenuation is set for the weld in dB/mm even if the reference point were not set to zero the

attenuation calculations would only confirm that the set attenuation is the actual attenuation.

Figure 29:A-scan of the CIVA simulation of weld 68 with 1 MHz, 0º probe.

The simulations do however show how much signal is left at the bottom of the weld. This can

therefore be used to make a rough estimation if the attenuation is so high that measurements

would be useless since the signal drop is to large.

Figure 30: The signal drop in the A-scan of the CIVA simulation of weld 68 with 1 MHz and 0º.

47

Simulations performed with the measured attenuation for 1 MHz and 0º indicates that if the

frequency is increased to 5 MHz all the signal is lost within the weld. And even though

attenuation is frequency dependent these types of simulations could give a good indication of

which frequencies are able to penetrate the weld with enough amplitude to be measurable.

Figure 31: Beam path of the CIVA simulation of weld 68 with 1 MHz, 0º probe.

5.3 Possible reasons for the deviation in attenuation

The difference in attenuation between the legs is of great importance since this difference

means that a technique used for measurements in one of the legs might not be as suitable for

the others.

48

Figure 32: A heat transient simulation of weld 68-1

The heat generated is concentrated in the leg and in the left corner of the test block as shown

in Figure 32. Since the test block is a mock-up of the real core shroud the left and right corner

next to the legs does not exist in reality. Since the simulations is based on equation 18, where

T is defined as the temperature gradient the different result in these simulations shows how

much the temperature gradient is affected by the edges of the test block.

49

Figure 33: A heat transient simulation of weld 68-2

Figure 33 shows a temperature development that is more similar to reality since there are no

corners in which the heat distribution is limited to. When comparing Figure 33 and Figure 34

that are supposed to show the same characteristics with only differences in defects, it becomes

clear that the temperature gradient is different in the legs.

50

Figure 34: A transient heat simulation of weld 68-3

The simulations are not a perfect representation of reality since the welds are quite large and

have probably been performed with multiple strings. But the first string should be done in a

fairly similar way to the simulations.

For weld 69 there is not the same amount of heat build-up at the edges. This is most likely due

to the fact that this is a longer weld, which helps the heat to distribute itself evenly as shown

in Figure 35.

51

Figure 35: Transient heat simulation of weld 69

The pump-deck (the vertical wall) does however affect the heat distribution. This wall allows

the heat to escape in two directions unlike the opposite side of the weld. This causes one of

the sides to cool quicker which also will affect the columnar grains. This could have an effect

on the angled probes when scanning since the columnar grains might lean in one direction in

the weld.

It should be noted that since the welds are constructed with many strings the welds will not

always be centred which is the setup in the simulation. Some of the string will be leaning

towards one of the sides meaning that the heat flow will have one primary direction in which

it flows. It is though possible that when welding upon a previous weld the temperature

becomes so high that recrystallization occurs. If so the new welds will determine the

properties of the old welds, and the order in which the welds are performed becomes a strong

factor in what the properties of the finished weld is.

The fact that the weld is created with so many strings could also be one of the reasons for the

difference in attenuation. Even though they are performed with the same welding parameters

and same positions. Some small deviation in angle, even though the position remains

constant, can change the placement of the weld. These differences are small, though since

there are many welds the differences could have an effect when they are added up. To test if

this phenomenon occurs the weld could be performed using a mechanical weld. This ensures

that the exact same welding procedure is carried out for every weld.

52

6. Conclusions

By performing transmission measurements on the test block the signal loss when sound

travels through the material can be measured. Then using the signal loss in the base material

and the welds material the attenuation can be calculated. By calculating the attenuation, the

scanning of the block can be simulated in CIVA. This can be used as an estimation of what

frequencies are possible to use when scanning the material. Since the attenuation is, among

other things, dependent on frequency this cannot be used as a precise method. This process

only works for 0º probes as the simulation software is unable to handle the transmission

simulations for angled probes due to alignment problems with the probes. Another factor that

limits the simulation possibilities is that the geometries of the weld. CIVA can only perform

transmission simulations on simple geometries such as rectangles and might not be able to

simulate other geometries.

The attenuation differs for the welds even though they are created with the same process and

parameters. This could be due to differences in the geometry of the test block. Since the

production cost of the test block is a big factor only a part of the core shroud is manufactured.

This can lead to corners being created where the real tank would be continuous. When

welding the tank, the heat distribution will therefore not be the same as in the real case and

therefore neither will the temperature gradient. If the temperature gradient differs there is a

chance that this could also have an effect on the columnar grain that is created in the

microstructure of the weld. Since the columnar grains is one of the major factors affecting the

attenuation, these small differences within the microstructure can cause a deviation in

attenuation between the welds that might make it harder to perform the actual site

measurements. The difference in the microstructure can also be due to small deviations in the

welding angle which arises since the welds are not performed mechanically.

53

7. Future work

7.1 Distinguish the primary cause of the columnar grain growth direction

There are many factors that can affect the microstructure and the attenuation of the welds. In

order to make sure which mechanisms that are dominate the effects, all of the variables should

be measured separate and thereby the understanding of the material can become greater.

By doing so the understanding of the attenuation problem would become greater and thereby

allowing an easier path to reduce the problem.

The focus of this research should be on if the welding position affects the microstructure and

the grain growth, and if so if there is a way of predicting in which direction the columnar

grains will be created in relation to the welding position. It should also be investigated if

edges close to the welding are the temperature gradients and thereby affecting the columnar

grain direction and the attenuation of the welds.

7.2 Enable angled simulations

It is important to be able to perform the simulations for the angled probes since these are very

common within the NDT ultrasonics. If it becomes possible to perform the simulation for

angled probes as well, it would make it possible to confirm that these type of simulation are

also possible to perform for angled probes.

CIVA are currently in the development of the transmission simulations and if it becomes

possible and the angled probes were developed these type of fast measurements and

simulations could be very helpful and cheaper than to perform time demanding

measurements.

54

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10. N. Bailey, F.R Coe, T.G Gooch, P.H.M Hart, N Jenkins, R.J PArgeter (1993) Welding

steels without hydrogen cracking (Second Edition)

11. A. Erhard, G. Schenk, W. Möhrle, HJ. Montag (2000) Ultrasonic phased array

technique for austenitic weld inspection

12. Welding positions according to EN ISO 6947 (2016-05-11)

https://www.messergroup.com/documents/20195/58291/Welding+positions+accordin

g+to+DIN+EN+ISO+6947/23711d16-8114-424a-bab3-40c257dd0170?version=1.1

13. A. B. Bhatia (1967) Ultrasonic absorption, an introduction to the theory of sound

absorption and dispersion in gases liquids and solids.

14. Peter B. Nagy (2001) Introduction to ultrasonics

15. Ji-Jung Kai and R.D. Lee (1992), Effects of irradiation on the microstructure of

INCONEL 600 alloy. Journal of Nuclear Materials, 191-194, 717-721

16. Sanjeevareddy Kolkoori, Mehubub-U Rahman, Jens Prager (2012) Effect of Columnar

Grain Orientation on Ultrasonic Plane Wave Energy Reflection and Transmission

Behaviour in Anisotropic Austenitic Weld Materials

17. Special Metals Products. (2016) INCONEL alloy 600.

http://www.specialmetals.com/documents/Inconel%20alloy%20600.pdf

18. A. Alippi and W.G Mayer (1987), Ultrasonic methods in evaluation of inhomogeneous

materials.

19. W. N. Reynolds And R. L. Smith (1983) Ultrasonic Wave attenuation in steels

20. A. Leger, M Deschamps (2009) Ultrasonic wave propagation in Non Homogeneous

Media

21. S.P Gornaja and N.P. Aljoshin (1995) Attenuation of ultrasonic waves in austenitic

steel.

22. Antonio Arnau (2008) Piezoelectric Transducers and Applications (Second Edition)

23. John Fuchs (2011) Ultrasonic – Transducers – Piezoelectric Effect

http://www.ctgclean.com/tech-blog/2011/12/ultrasonics-transducers-piezoelectric-

effect/ retrieved 04-02-2016

55

24. Pietro Burrascano, Sergio Callegari, Audusto Montisci, Marco Ricci, Mario Versaci

(2015) Ultrasonic Nondestructive Evaluation Systems Industrial applications issues

25. P. Calmon, S. Mahuat, S Chatillon, R. Raillon (2006) CIVA An expertise platform for

simulation and processing NDT data

26. Nicolas Gengembre, Alain Lhémery (2000) Pencil method in elastodynamics:

application to ultrasonic field computation

27. Steve Mahuat, Sylvian Chatillon, Nicolas Leymarie,

Frédéric Jenson and Pierre Calmon (2007) Simulation tools for predicting non-

destructive testing of heterogeneous and anisotropic structures

28. ANSYS Thermal Analysis Guide

56

Appendix A

Blueprint of testblock

57

58

59

60

61

Appendix B

ANSYS Simulation setup

Project

First Saved Thursday, April 14, 2016

Last Saved Monday, May 9, 2016

Product Version 17.0 Release

Save Project Before Solution No

Save Project After Solution No

Contents

l Units

l Model (A4) ¡

Ge

62

o

m

etr

y n

So

lid ¡ Coordinate Systems ¡ Mesh

n Body Sizing ¡ Transient Thermal (A5) n Initial

Temperature n Analysis

Settings n Temperature n

Solution (A6) n Solution

Information n Result Charts n Temperature

l Material Data ¡ Structural Steel

Units

TABLE 1

Unit System Metric (m, kg, N, s, V, A) Degrees rad/s Celsius

Angle Degrees

Rotational Velocity rad/s

Temperature Celsius

Model (A4)

Geometry

TABLE 2 Model (A4) > Geometry

Object Name Geometry

State Fully Defined

Definition

Source C:\Users\kaffe\Desktop\Model\ANSYS\W68_files\dp0

\SYS\DM\SYS.agdb

Type DesignModeler

Length Unit Meters

Element Control Program Controlled

Display Style Body Color

Bounding Box

Length X 1,0081 m

Length Y 1,14 m

Length Z 0,44126 m

Properties

Volume 7,8029e-002 m³

63

Mass 612,53 kg

Scale Factor Value 1,

Statistics

Bodies 1

Active Bodies 1

Nodes 30781

Elements 18122

Mesh Metric None

Basic Geometry Options

Solid Bodies Yes

Surface Bodies Yes

Line Bodies Yes

Parameters Yes

Parameter Key

Attributes Yes

Attribute Key

Named Selections Yes

Named Selection Key

Material Properties Yes

Advanced Geometry Options

Use Associativity Yes

Coordinate Systems Yes

Reader Mode Saves Updated File No

Use Instances Yes

Smart CAD Update Yes

Compare Parts On Update No

Attach File Via Temp File Yes

Temporary Directory C:\Users\kaffe\AppData\Local\Temp

Analysis Type 3-D

Mixed Import Resolution None

Decompose Disjoint Geometry Yes

Enclosure and Symmetry Processing Yes

TABLE 3 Model (A4) > Geometry > Parts

Object Name Solid

State Meshed

Graphics Properties

Visible Yes

Transparency 1

Definition

Suppressed No

Stiffness Behavior Flexible

64

Coordinate System Default Coordinate System

Reference Temperature By Environment

Behavior None

Material

Assignment Structural Steel

Nonlinear Effects Yes

Thermal Strain Effects Yes

Bounding Box

Length X 1,0081 m

Length Y 1,14 m

Length Z 0,44126 m

Properties

Volume 7,8029e-002 m³

Mass 612,53 kg

Centroid X 0,96502 m

Centroid Y 0,56908 m

Centroid Z 9,8975e-002 m

Moment of Inertia Ip1 73,247 kg·m²

Moment of Inertia Ip2 43,678 kg·m²

Moment of Inertia Ip3 100,13 kg·m²

Statistics

Nodes 30781

Elements 18122

Mesh Metric None

Coordinate Systems

TABLE 4 Model (A4) > Coordinate Systems > Coordinate System

Object Name Global Coordinate System

State Fully Defined

Definition

Type Cartesian

Coordinate System ID 0,

Origin

Origin X 0, m

Origin Y 0, m

Origin Z 0, m

Directional Vectors

X Axis Data [ 1, 0, 0, ]

Y Axis Data [ 0, 1, 0, ]

Z Axis Data [ 0, 0, 1, ]

Mesh

65

TABLE 5 Model (A4) > Mesh

Object Name Mesh

State Solved

Display

Display Style Body Color

Defaults

Physics Preference Mechanical

Relevance 0

Shape Checking Standard Mechanical

Element Midside Nodes Program Controlled

Sizing

Size Function Adaptive

Relevance Center Coarse

Element Size Default

Initial Size Seed Active Assembly

Smoothing Medium

Transition Fast

Span Angle Center Coarse

Automatic Mesh Based Defeaturing On

Defeaturing Tolerance Default

Minimum Edge Length 5,e-005 m

Inflation

Use Automatic Inflation None

Inflation Option Smooth Transition

Transition Ratio 0,272

Maximum Layers 5

Growth Rate 1,2

Inflation Algorithm Pre

View Advanced Options No

Advanced

Number of CPUs for Parallel Part Meshing Program Controlled

Straight Sided Elements No

Number of Retries Default (4)

Extra Retries For Assembly Yes

Rigid Body Behavior Dimensionally Reduced

Mesh Morphing Disabled

Triangle Surface Mesher Program Controlled

Topology Checking No

Pinch Tolerance Please Define

Generate Pinch on Refresh No

Statistics

66

Nodes 30781

Elements 18122

Mesh Metric None

TABLE 6 Model (A4) > Mesh > Mesh Controls

Object Name Body Sizing

State Fully Defined

Scope

Scoping Method Geometry Selection

Geometry 1 Body

Definition

Suppressed No

Type Element Size

Element Size 3,5e-002 m

Behavior Soft

Transient Thermal (A5)

TABLE 7 Model (A4) > Analysis

Object Name Transient Thermal (A5)

State Solved

Definition

Physics Type Thermal

Analysis Type Transient

Solver Target Mechanical APDL

Options

Generate Input Only No TABLE 8

Model (A4) > Transient Thermal (A5) > Initial Condition

Object Name Initial Temperature

State Fully Defined

Definition

Initial Temperature Uniform Temperature

Initial Temperature Value 22, °C

TABLE 9 Model (A4) > Transient Thermal (A5) > Analysis Settings

Object Name Analysis Settings

State Fully Defined

Step Controls

Number Of Steps 1,

Current Step Number 1,

Step End Time 2000, s

Auto Time Stepping Program Controlled

Initial Time Step 20, s

67

Minimum Time Step 2, s

Maximum Time Step 200, s

Time Integration On

Solver Controls

Solver Type Program Controlled

Radiosity Controls

Radiosity Solver Program Controlled

Flux Convergence 1,e-004

Maximum Iteration 1000,

Solver Tolerance 0,1 W/m²

Over Relaxation 0,1

Hemicube Resolution 10,

Nonlinear Controls

Heat Convergence Program Controlled

Temperature Convergence Program Controlled

Line Search Program Controlled

Nonlinear Formulation Program Controlled

Output Controls

Calculate Thermal Flux Yes

General Miscellaneous No

Store Results At All Time Points

Analysis Data Management

Solver Files Directory C:\Users\kaffe\Desktop\Model\ANSYS\W68_files\dp0\SYS\MECH\

Future Analysis None

Scratch Solver Files Directory

Save MAPDL db No

Delete Unneeded Files Yes

Nonlinear Solution No

Solver Units Active System

Solver Unit System mks TABLE 10

Model (A4) > Transient Thermal (A5) > Loads

Object Name Temperature

State Fully Defined

Scope

Scoping Method Geometry Selection

Geometry 2 Faces

Definition

Type Temperature

Magnitude Tabular Data

Suppressed No

Tabular Data

Independent Variable Time FIGURE 1

68

Model (A4) > Transient Thermal (A5) > Temperature

TABLE 11 Model (A4) > Transient Thermal (A5) > Temperature

Steps Time [s] Temperature [°C]

1 0, 1600,

2000, = 1442,2

N/A 20000 22,

Solution (A6)

TABLE 12 Model (A4) > Transient Thermal (A5) > Solution

Object Name Solution (A6)

State Solved

Adaptive Mesh Refinement

Max Refinement Loops 1,

Refinement Depth 2,

Information

Status Done

MAPDL Elapsed Time 13, s

MAPDL Memory Used 158, MB

MAPDL Result File Size 83,188 MB

Post Processing

Calculate Beam Section Results No

TABLE 13 Model (A4) > Transient Thermal (A5) > Solution (A6) > Solution Information

69

Object Name Solution Information

State Solved

Solution Information

Solution Output Solver Output

Update Interval 2,5 s

Display Points All

FE Connection Visibility

Activate Visibility Yes

Display All FE Connectors

Draw Connections Attached To All Nodes

Line Color Connection Type

Visible on Results No

Line Thickness Single

Display Type Lines TABLE 14

Model (A4) > Transient Thermal (A5) > Solution (A6) > Solution Information > Result Charts

Object Name Temperature - Global Maximum Temperature - Global Minimum

State Solved

Definition

Type Temperature

Suppressed No

Scope

Scoping Method Global Maximum Global Minimum

Results

Minimum 1442,2 °C 22, °C

Maximum 1598,4 °C 22,019 °C

FIGURE 2 Model (A4) > Transient Thermal (A5) > Solution (A6) > Solution Information > Temperature - Global

Maximum

70

FIGURE 3 Model (A4) > Transient Thermal (A5) > Solution (A6) > Solution Information > Temperature - Global

Minimum

TABLE 15

71

Model (A4) > Transient Thermal (A5) > Solution (A6) > Results

Object Name Temperature

State Solved

Scope

Scoping Method Geometry Selection

Geometry All Bodies

Definition

Type Temperature

By Time

Display Time Last

Calculate Time History Yes

Identifier

Suppressed No

Results

Minimum 22,019 °C

Maximum 1442,2 °C

Minimum Occurs On Solid

Maximum Occurs On Solid

Minimum Value Over Time

Minimum 22, °C

Maximum 22,019 °C

Maximum Value Over Time

Minimum 1442,2 °C

Maximum 1598,4 °C

Information

Time 2000, s

Load Step 1

Substep 18

Iteration Number 18

FIGURE 4

Project Page 1 of 74

file:///C:/Users/kaffe/AppData/Roaming/Ansys/v170/Mechanical_Report/Mechanical... 2016-

05-09

Model (A4) > Transient Thermal (A5) > Solution (A6) > Temperature

TABLE 16 Model (A4) > Transient Thermal (A5) > Solution (A6) > Temperature

Time [s] Minimum [°C] Maximum [°C]

20,

22,

1598,4

27,509 1597,8

35,018 1597,2

49,141 1596,1

71,282 1594,4

104,83 1591,7

156,35 1587,7

239,56 1581,1

382,91 1569,8

582,91 1554,

782,91 1538,2

982,91 1522,4

1182,9 22,001 1506,7

1382,9 22,002 1490,9

1582,9 22,005 1475,1

1782,9 22,01 1459,3

1891,5 22,014 1450,8

2000, 22,019 1442,2

Material Data

2

Structural Steel

TABLE 17 Structural Steel > Constants

Density 7850, kg m^-3

Coefficient of Thermal Expansion 1,2e-005 C^-1

Specific Heat 434, J kg^-1 C^-1

Thermal Conductivity 60,5 W m^-1 C^-1

Resistivity 1,7e-007 ohm m

TABLE 18 Structural Steel > Color

Red Green Blue

132, 139, 179,

TABLE 19 Structural Steel > Compressive Ultimate Strength

Compressive Ultimate Strength Pa

0, TABLE 20

Structural Steel > Compressive Yield Strength

Compressive Yield Strength Pa

2,5e+008 TABLE 21

Structural Steel > Tensile Yield Strength

Tensile Yield Strength Pa

2,5e+008

TABLE 22 Structural Steel > Tensile Ultimate Strength

Tensile Ultimate Strength Pa

4,6e+008

TABLE 23

3

Structural Steel > Isotropic Secant Coefficient of Thermal Expansion

Zero-Thermal-Strain Reference Temperature C

22,

TABLE 24 Structural Steel > Alternating Stress Mean Stress

Alternating Stress Pa Cycles Mean Stress Pa

3,999e+009 10, 0,

2,827e+009 20, 0,

1,896e+009 50, 0,

1,413e+009 100, 0,

1,069e+009 200, 0,

4,41e+008 2000, 0,

2,62e+008 10000 0,

2,14e+008 20000 0,

1,38e+008 1,e+005 0,

1,14e+008 2,e+005 0,

8,62e+007 1,e+006 0, TABLE 25

Structural Steel > Strain-Life Parameters

Strength

Coefficient Pa Strength

Exponent Ductility

Coefficient Ductility

Exponent Cyclic Strength

Coefficient Pa Cyclic Strain

Hardening Exponent

9,2e+008 -0,106 0,213 -0,47 1,e+009 0,2

TABLE 26 Structural Steel > Isotropic Elasticity

Temperature C Young's Modulus Pa Poisson's Ratio Bulk Modulus Pa Shear Modulus Pa

2,e+011 0,3 1,6667e+011 7,6923e+010

TABLE 27 Structural Steel > Isotropic Relative Permeability

Relative Permeability

10000

TRITA FYS 2016:34

ISSN 0280-316X

ISRN KTH/FYS/--16:34—SE

www.kth.se